1. Field of the Invention
The present invention relates to high-speed data communications cables comprising at least two twisted pairs of insulated conductors. More particularly, the invention relates to high-speed data communications cables that may be exposed to force, stress, rough handing and/or other disturbances present in mechanically dynamic environments.
2. Discussion of the Related Art
High-speed data communications cables often include pairs of insulated conductors twisted together generally in a double-helix pattern about a longitudinal axis. Such an arrangement of insulated conductors, referred to herein as “twisted pairs,” facilitates forming a balanced transmission line for data communications. One or more twisted pairs may subsequently be bundled and/or bound together to form a data communications cable.
A cable may undergo various mechanical stresses during handling and use. For example, cables may be exposed to rough handling during installation of a structured cabling architecture for a local area network (LAN), during cable pulling and tying, etc. In addition, cables may be employed in various industrial settings wherein the cable is likely to be subjected to often rigorous motion, various mechanical stresses such as bending and twisting, and/or general rough handling during ordinary use.
One example of relatively harsh treatment of cables occurs in automatic cable dispensing devices. In order to facilitate cable deployment and/or installation, a cable may be packaged and distributed in a container or housing having various mechanical features that automatically dispense cable during installation. Such housings are generally desirable with respect to simplifying and expediting cable deployment. However, the automatic features of such devices often apply forces and various mechanical stresses to the cable during operation. Such relatively harsh treatment may alter the configuration and/or arrangement of the twisted pairs making up the cable.
The Telecommunications Industry Association and the Electronics Industry Association (TIA/EIA) have developed standards specifying a number of performance categories that establish requirements for various operating characteristics of a cable. For example, a category 6 cable must meet requirements for cable impedance and return loss, signal attenuation and delay, crosstalk, etc. A category 6 cable is generally considered a high performance cable and, as such, return loss and crosstalk requirements may be particularly stringent.
The term “return loss” refers to a measure of the relationship between the transmitted electrical energy and reflected electrical energy along a transmission line (e.g., a data communications cable). For example, return loss may be measured as the ratio of the signal power transmitted into a system (e.g., the power generated at the source end of a cable) to the signal power that is reflected. Return loss is often indicated in decibel (dB) units. Reflected electrical energy may have various adverse effects on data transmission, including reduced output power, signal distortion and dispersion, signal loss (e.g., attenuation), etc. The severity of return loss effects may depend on frequency. For example, high frequency signals tend to be more sensitive to distortion effects associated with return loss. The return loss requirements for category 6 cables may therefore be rated in connection with transmission signal frequency. Accordingly, higher performance cables may be more vulnerable to return loss effects caused by rough handling of the cables.
A variety of factors may contribute to generating reflections that affect the return loss of a cable. For example, an impedance mismatch between a cable and a load that is coupled to the cable may cause reflections that adversely affect return loss. Other reflections may stem from unintended variation in cable properties, non-uniformities and/or discontinuities along the length of a cable. Mechanical stresses on conventional cables in mechanically dynamic environments may result in variation in the intended lay configuration of the cable which may degrade the cable's return loss characteristics such that the cable no longer meets the performance requirements of its intended category.
Referring to FIG. 1A, there is illustrated a perspective view of a twisted pair of insulated conductors 50. Twisted pair 50 may be one of a plurality of twisted pairs bundled together to form a data communications cable. Twisted pair 50 comprises a pair of conductors 60a and 60b, respectively insulated by insulators 62a and 62b. Ideally, the two insulated conductors making up twisted pair 50 should be in contact or maintain a uniform spacing or air gap along the entire twisted length of twisted pair 50. However, various factors, such as rough handling and/or a tendency of the insulated conductors to untwist may cause some separation between the two conductors at various points along the length of the twisted pair. For example, at a length L1 along a longitudinal axis 64 of the twisted pair 50, the twisted pair may be positioned as intended with the insulators 62a and 62b in contact with one another. FIG. 1B is a cross-sectional diagram of the twisted pair 50 at length L1, taken along line B—B. As illustrated in FIG. 1B, in such an arrangement, respective centers of conductors 60a and 60b are separated by a distance d1, determined at least in part by the diameter of the conductors and the thickness of the insulators. This distance is referred to herein as the “center-to-center distance.”
A characteristic impedance of twisted pair 50 may be related to several parameters including the diameter of the conductors 60a and 60b, the center-to-center distance, the dielectric constant of insulators 62a and 62b, etc. In order to impedance match a cable to a load (e.g., a network component), the cable may be rated with a particular characteristic impedance. For example, many radio frequency (RF) components may have characteristic impedances of 50, 75 or 100 Ohms. Therefore, many high frequency cables may similarly be rated with a characteristic impedance of 50, 75 or 100 Ohms so as to facilitate connecting of different RF loads. Often, the characteristic impedance is determined from the average impedance of the cable based on the intended arrangement (i.e., arrangements wherein the insulators are in contact or have a uniform, controlled air gap between them), as illustrated at length L1 in FIGS. 1A and 1B. However, referring again to FIG. 1A, as discussed above, at a length L2 along the longitudinal axis 64, the center-to-center spacing between conductors of the pair may separate or compress to some extent such that the insulators 62a, 62b no longer have the intended spacing due to, for example, bending, twisting and/or other rough handling of the cable. Accordingly, the center-to-center distance has increased to a distance d2, as shown in FIG. 1C which is a cross-sectional diagram of the twisted pair taken along line C—C in FIG. 1A. At some arbitrary length L3 (see FIG. 1A), the twisted pair 50 may have yet another different center-to-center distance between the two conductors. This variation in the center-to-center distance may cause the impedance of the twisted pair to vary along the length of the twisted pair 50, resulting in undesirable signal reflections that affect return loss.
In addition, when the insulators of a twisted pair are not in contact, the dielectric between the two conductors includes an amount of air, the amount depending on the extent of the separation. As a result, the dielectric composition of the twisted pair may vary along the longitudinal length of the twisted pair, causing further variation characteristic impedance of the twisted pair that may, in turn, produce unwanted signal reflections that degrade the return loss of the cable.